Solid golf ball

ABSTRACT

The invention provides a solid golf ball having a core and a cover, the core being formed of a rubber composition containing a base rubber, a co-crosslinking agent, a crosslinking initiator and a metal oxide. The base rubber is a mixture of polybutadiene and a styrene-butadiene rubber, the styrene-butadiene rubber having a styrene bond content of not more than 35 wt %. The co-crosslinking agent is methacrylic acid. The core deflection under specific loading and the VR value for dimples on the ball surface are set within specific ranges.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending application Ser. No. 13/351,790 filed on Jan. 17, 2012, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a golf ball for long-term use, and more specifically to a golf ball which has an excellent durability to cracking and durability of appearance, which is able to maintain a stable feel at impact and a stable flight performance over an extended period of time, and which has a controlled distance.

In order to maintain the durability of a golf ball in long-term use, it is necessary to enhance the durability of each member of the ball and the wear resistance of the outside surface. It is also necessary to maintain the performance of the golf ball in different seasons.

As is widely known, two-piece solid golf balls are composed of a core and a cover, with the core being a crosslinked rubber structure of certain desirable properties obtained by using a base rubber composed primarily of cis-1,4-polybutadiene rubber to which compounding ingredients such as a co-crosslinking agent, a metal oxide and an organic peroxide have been added. For example, JP-A 59-49779 describes a rubber composition for the core of a two-piece solid golf ball which is obtained by compounding a given amount of zinc methacrylate as a co-crosslinking agent in cis-1,4-polybutadiene rubber. However, when zinc methacrylate is used in this way in a core-forming rubber composition, ensuring good ball durability in long-term use has been difficult.

In addition, JP-A 2003-70936 describes a rubber composition for the core of a two-piece solid golf ball which is obtained by compounding a given amount of zinc acrylate in cis-1,4-polybutadiene rubber. However, here too, when zinc acrylate is used in the rubber-forming rubber composition, ensuring good ball durability in long-term use has been difficult.

Also, JP-A 2004-180793 and JP-A 2008-149190 disclose golf balls which use zinc acrylate in the core formulation and use a thermoplastic polyurethane as the cover material. However, drawbacks of such golf balls include a hard cover, a marked decrease in ball flight following abrasion of the ball surface, and poor durability of markings.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a solid golf ball which has an excellent durability to cracking, durability of appearance and durability to ball surface loss in long-term use, which maintains a stable feel at impact and a stable flight performance over an extended period of time, and which has a controlled distance.

As a result of extensive investigations, the inventors have discovered that, in the fabrication of a solid golf ball having a core and a cover, by using a mixture of polybutadiene and a styrene-butadiene rubber as the base rubber and optimizing the styrene bond content in the styrene-butadiene rubber, by using methacrylic acid as a co-crosslinking agent and including a specific amount of a crosslinking initiator per 100 parts by weight of the base rubber, by optimizing the deflection of the core under a specific load, the initial velocity of the finished ball, and the dimple spatial occupancy VR, and by using a resin material having a breaking strength of from 20 to 80 MPa and an elongation of from 150 to 600% as the cover material, owing to synergistic effects from these constituent features, the resulting ball is endowed with an excellent durability to cracking, durability to surface loss and durability to abrasion that exceed the expectations of golf ball designers. As a result, there can be obtained a golf ball which, even in long-term use, maintains a good appearance, has a good feel at impact and has a controlled distance.

That is, in the present invention, by including methacrylic acid as a co-crosslinking agent in the core-forming rubber composition, and by optimizing the amount of methacrylic acid and the amount of crosslinking initiator, the durability to cracking can be made much better than that in game balls. Moreover, having the cover material composed primarily of polyurethane is desirable in that it enables a golf ball of excellent durability to cracking and durability to abrasion to be obtained. In addition, optimizing the internal hardness profile of the core is desirable in that it enables a solid golf ball having a good feel at impact to be obtained.

Accordingly, the invention provides the following solid golf ball.

[1] A solid golf ball comprising a core and a cover, the core being formed of a rubber composition comprising a base rubber, a co-crosslinking agent, a crosslinking initiator and a metal oxide, wherein the base rubber is a mixture of polybutadiene and a styrene-butadiene rubber, the styrene-butadiene rubber having a styrene bond content of not more than 35 wt %, and the co-crosslinking agent is methacrylic acid; the core has a deflection CH when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) of from 2.5 to 7.0 mm; and the ball has formed on a surface thereof a plurality of dimples, each having a spatial volume below a flat plane circumscribed by an edge of the dimple, the sum of the individual dimple spatial volumes, expressed as a percentage (VR) of the volume of a hypothetical sphere were the ball to have no dimples on the surface thereof, being from 0.95 to 1.7. [2] The solid golf ball of [1], wherein the metal oxide is zinc oxide. [3] The solid golf ball of [1], wherein the polybutadiene accounts for up to 80 wt % of the base rubber in the rubber composition, the styrene-butadiene rubber accounts for between 20 and 80 wt % of the base rubber, and the isoprene rubber accounts for between 0 and 60 wt % of the base rubber; and wherein the rubber composition includes from 6 to 40 parts by weight of methacrylic acid, from 6 to 30 parts by weight of the metal oxide, from 0.3 to 5.0 parts by weight of the crosslinking initiator, and from 0.1 to 1.0 part by weight of the antioxidant per 100 parts by weight of the base rubber. [4] The solid golf ball of [1], wherein the core has a specific gravity of from 1.05 to 1.2. [5] The solid golf ball of [1], wherein the cover is formed of a resin material which is composed primarily of a polyurethane. [6] The solid golf ball of [5], wherein the resin material of the cover is composed primarily of a thermoplastic polyurethane. [7] The solid golf ball of [1], wherein the cover has a material hardness, expressed in terms of Shore D hardness, of from 30 to 57. [8] The solid golf ball of [1], wherein the cover is formed of a resin material having a breaking strength of from 20 to 80 MPa. [9] The solid golf ball of [1], wherein the cover is formed of a resin material having an elongation of from 150 to 600%. [10] The solid golf ball of [1], wherein the cover has a thickness of from 0.3 to 2.5 mm. [11] The solid golf ball of [1], wherein the ball has an initial velocity (BV) of not more than 72 m/s. [12] The solid golf ball of [1], wherein the core has a deflection CH (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), the ball has, upon initial measurement, a deflection BH1 (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and an initial velocity BV1 (m/s), and also has, when measured again after 350 days of standing following initial measurement, a deflection BH2 (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and an initial velocity BV2 (m/s), such that: BH1 is from 2.5 to 7.0 mm, the ratio CH/BH1 is from 0.95 to 1.1, the difference BH2−BH1 is not more than 0.2 mm, and the difference BV2−BV1 is not more than 0.3 m/s. [13] The solid golf ball of [1], wherein the dimples formed on the surface of the ball satisfy conditions (1) to (6) below:

(1) the dimples have a peripheral edge provided with a roundness represented by a radius of curvature R of from 0.5 to 2.5 mm;

(2) the ratio ER of a collective number of dimples RA having a radius of curvature R to diameter D ratio (R/D) of at least 20%, divided by a total number of dimples N on the surface of the ball, is from 15 to 95%;

(3) the ball has thereon a plurality of dimple types of differing diameter, and the ratio DER of a combined number of dimples DE obtained by adding together dimples having an own diameter and an own radius of curvature larger than or equal to a radius of curvature of dimples of larger diameter than said own diameter plus dimples of a type having a largest diameter, divided by the total number of dimples N on the surface of the ball, is at least 80%;

(4) the number of dimple types of differing diameter is 3 or more;

(5) the total number of dimples N is not more than 380; and

(6) the surface coverage SR of the dimples, which is the sum of individual dimple surface areas, each defined by a flat plane circumscribed by an edge of the dimple, expressed as a percentage of the surface area of a hypothetical sphere were the ball to have no dimples on the surface thereof, is from 60 to 74%.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a schematic cross-sectional diagram of a solid golf ball according to one embodiment of the invention.

FIG. 2 is a schematic diagram of a core illustrating positions A to F in a core hardness profile.

FIG. 3 is a schematic diagram showing an example of a dimple cross-section.

FIG. 4A is a top view and FIG. 4B is a side view showing an example of a dimple configuration.

FIG. 5 is a top view showing the markings that were placed on the golf balls fabricated in the examples and the comparative examples.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described more fully below.

The solid golf ball of the invention has a structure which is exemplified by, as shown in FIG. 1, a two-piece solid golf ball G having a core 1 and a cover 2 that encases the core. The cover 2 has a surface on which, typically, a plurality of dimples D are formed. In the diagram, the core 1 and the cover 2 are each formed as single layers, although either or both may be composed of a plurality of layers.

The core is obtained by vulcanizing a rubber composition composed primarily of a rubber material. The rubber composition used to form the core includes a base rubber, a co-crosslinking agent, a crosslinking initiator, a metal oxide, an antioxidant and, optionally, an inert filler. In the invention, a mixture of polybutadiene with a styrene-butadiene rubber is used as the base rubber. Also, in the present invention, as will be subsequently described, it is preferable for the core cross-sectional hardness to change in specific ways from the surface to the center of the core, and for the core cross-sectional hardness profile to be adjusted within certain desired ranges. To this end, in formulating the core, it is essential to suitably adjust, for example, the amounts in which the various subsequently described compounding ingredients are included, the vulcanization temperature and the vulcanization time.

The polybutadiene used as a rubber component must have a cis-1,4 bond content of at least 60 wt %, preferably at least 80 wt %, more preferably at least 90 wt %, and most preferably at least 95 wt %. If the cis-1,4 bond content is too low, the rebound may decrease. In addition, the polybutadiene has a 1,2-vinyl bond content of preferably 2 wt % or less, more preferably 1.7 wt % or less, and even more preferably 1.5 wt % or less.

The polybutadiene has a Mooney viscosity (ML₁₊₄ (100° C.)) which is preferably at least 30, more preferably at least 35, and even more preferably at least 40. The upper limit is preferably not more than 100, more preferably not more than 80, even more preferably not more than 70, and most preferably not more than 60.

The term “Mooney viscosity” used herein refers to an industrial indicator of viscosity (JIS K6300) as measured with a Mooney viscometer, which is a type of rotary plastometer. This value is represented by the unit symbol ML₁₊₄ (100° C.), wherein “M” stands for Mooney viscosity, “L” stands for large rotor (L-type), and “1+4” stands for a pre-heating time of 1 minute and a rotor rotation time of 4 minutes. The “100° C.” indicates that measurement was carried out at a temperature of 100° C.

In order to obtain the rubber composition in a molded and vulcanized form which has a good rebound, it is preferable for the polybutadiene to have been synthesized using a rare-earth catalyst or a Group VIII metal compound catalyst.

The rare-earth catalyst is not subject to any particular limitation, although preferred use may be made of a catalyst which employs a lanthanum series rare-earth compound. Also, where necessary, an organoaluminum compound, an alumoxane, a halogen-bearing compound and a Lewis base may be used in combination with the lanthanum series rare-earth compound. Preferred use can be made of, as the various above compounds, those compounds mentioned in JP-A 11-35633, JP-A 11-164912 and JP-A 2002-293996.

Of the above rare-earth catalysts, the use of a neodymium catalyst that employs a neodymium compound, which is a lanthanide series rare-earth compound, is especially recommended. In such a case, a polybutadiene rubber having a high cis-1,4 bond content and a low 1,2-vinyl bond content can be obtained at an excellent polymerization activity.

The polybutadiene has a molecular-weight distribution Mw/Mn (Mw being the weight-average molecular weight, and Mn being the number-average molecular weight) of preferably at least 1.0, more preferably at least 2.0, even more preferably at least 2.2, and most preferably at least 2.4. The upper limit is preferably 6.0 or less, more preferably 5.0 or less, and even more preferably 4.5 or less. If Mw/Mn is too low, the workability may decrease. On the other hand, if Mw/Mn is too high, the rebound may decrease.

The above polybutadiene used in the base rubber accounts for a proportion of the overall base rubber which, although not subject to any particular limitation, is preferably not more than 80 wt %, more preferably not more than 70 wt %, even more preferably not more than 60 wt %, and most preferably not more than 57 wt %. The lower limit is preferably at least 30 wt %, more preferably at least 35 wt %, and even more preferably at least 38 wt %.

Illustrative examples of cis-1,4-polybutadiene rubbers which may be used include the high-cis products BR01, BR11, BR02, BR02L, BR02LL, BR730 and BR51, all of which are available from JSR Corporation.

In the present invention, a styrene-butadiene rubber (SBR) is used together with the above polybutadiene rubber (BR) as the base rubber. The styrene-butadiene rubber is described below.

A solution-polymerized styrene-butadiene rubber or an emulsion-polymerized styrene-butadiene rubber may be used as the styrene-butadiene rubber (SBR). For example, use may be made of the solution-polymerized products SBR-SL552, SBR-SL555 and SBR-SL563 (available from JSR Corporation) as the solution-polymerized styrene-butadiene rubber, and use may be made of the emulsion-polymerized products SBR 1500, SBR 1502 and SBR 1507 (available from JSR Corporation) as the emulsion-polymerized styrene-butadiene rubber.

The styrene bond content in the styrene-butadiene rubber is preferably at least 5 wt %, more preferably at least 10 wt %, even more preferably at least 15 wt %, and most preferably at least 18 wt %. The upper limit is preferably not more than 35 wt %, more preferably not more than 30 wt %, even more preferably not more than 25 wt %, and most preferably not more than 22 wt %. If the styrene bond content is too high, due to temperature changes on account of seasonal differences, the core will become harder and large changes will occur in the rebound. On the other hand, if the styrene bond content is too low, the ease of operation during surface grinding of the core will dramatically decrease. Also, if the styrene bond content is too high, the scuff resistance at low temperature may worsen.

The styrene-butadiene rubber accounts for a proportion of the overall base rubber which is preferably at least 20 wt %, more preferably at least 25 wt %, even more preferably at least 30 wt %, and most preferably at least 35 wt %. The upper limit is preferably not more than 80 wt %, more preferably not more than 70 wt %, even more preferably not more than 60 wt %, and most preferably not more than 57 wt %.

Rubber ingredients other than the above polybutadiene and styrene-butadiene rubber (SBR) may also be included in the base rubber, insofar as the objects of the invention are attainable. Illustrative examples of rubber ingredients other than the above polybutadiene and styrene-butadiene rubber (SBR) include polybutadienes other than the above polybutadiene, and other diene rubbers such as natural rubbers, isoprene rubbers (IR) and ethylene-propylene-diene rubbers.

Isoprene rubbers (IR) which may be used include those having a cis-1,4 bond content of at least 60 wt %, preferably at least 80 wt %, and more preferably at least 90 wt %, and having a Mooney viscosity (ML₁₊₄ (100° C.)) of at least 60, preferably at least 70, and more preferably at least 80, with an upper limit of not more than 90, and preferably not more than 85. For example, the product IR2200 available from JSR Corporation may be used. The proportion of the overall base rubber represented by rubber ingredients other than polybutadiene is preferably more than 0 wt %, more preferably at least 2 wt %, and most preferably at least 5 wt %. The upper limit is preferably not more than 60 wt %, more preferably not more than 40 wt %, even more preferably not more than 20 wt %, and most preferably not more than 10 wt %.

In the invention, methacrylic acid is an essential ingredient which is used as the co-crosslinking agent. Methacrylic acid is included in an amount, per 100 parts by weight of the base rubber, of preferably at least 6 parts by weight, more preferably at least 8 parts by weight, even more preferably at least 10 parts by weight, and most preferably at least 11.5 parts by weight. The upper limit in the amount of methacrylic acid is preferably not more than 40 parts by weight, more preferably not more than 35 parts by weight, even more preferably not more than 30 parts by weight, and most preferably not more than 25 parts by weight. Including too much co-crosslinking agent may make the core too hard, giving the ball an unpleasant feel at impact. On the other hand, including too little co-crosslinking agent may make the core too soft, also giving the ball an unpleasant feel at impact.

It is preferable to use an organic peroxide as the crosslinking initiator. Examples of commercial products that may be advantageously used include Percumyl D (from NOF Corporation), Perhexa C40 (NOF Corporation) and Trigonox 29-40b (Akzo Nobel N.V.). These may be used singly or as a combination of two or more thereof.

The amount of crosslinking initiator per 100 parts by weight of the base rubber may be set to preferably at least 0.3 part by weight, more preferably at least 0.5 part by weight, and even more preferably at least 0.7 part by weight. The upper limit in the amount of crosslinking initiator may be set to preferably not more than 5.0 parts by weight, more preferably not more than 4.0 parts by weight, even more preferably not more than 3.0 parts by weight, and most preferably not more than 2.0 parts by weight. Including too much crosslinking initiator may make the core too hard, giving the ball an unpleasant feel at impact and also substantially lowering the durability to cracking. On the other hand, including too little crosslinking initiator may make the core too soft, giving the ball an unpleasant feel at impact and also substantially lowering productivity.

Although not subject to any particular limitation, in this invention, the use of zinc oxide as the metal oxide is preferred. The use of metal oxides other than zinc oxide is also possible, insofar as the objects of the invention are attainable. The metal oxide is included in an amount, per 100 parts by weight of the base rubber, of preferably at least 6 parts by weight, more preferably at least 8 parts by weight, even more preferably at least 10 parts by weight, and most preferably at least 12 parts by weight. The upper limit in the amount of metal oxide is preferably not more than 30 parts by weight, more preferably not more than 28 parts by weight, even more preferably not more than 26 parts by weight, and most preferably not more than 24 parts by weight. Including too much or too little metal oxide may make it impossible to obtain a suitable weight and a good hardness and rebound.

In the practice of the invention, it is preferable to include an antioxidant in the rubber composition. For example, use may be made of the commercial products Nocrac NS-6, Nocrac NS-30 and Nocrac 200 (all available from Ouchi Shinko Chemical Industry Co., Ltd.). These may be used singly or as combinations of two or more thereof.

The amount of antioxidant included per 100 parts by weight of the base rubber, although not subject to any particular limitation, is preferably at least 0.1 part by weight, and more preferably at least 0.15 part by weight, but is preferably not more than 1.0 part by weight, more preferably not more than 0.7 part by weight, and even more preferably not more than 0.4 part by weight. Including too much or too little antioxidant may make it impossible to achieve a suitable core hardness gradient, as a result of which a good rebound, good durability and good spin rate-lowering effect on full shots may not be achieved.

Preferred use may be made of, for example, barium sulfate, calcium carbonate or silica as the inert filler. Any one of these may be used alone or two or more may be used in combination. The amount of inert filler included is not particularly limited, although this amount is preferably more than 0, and may be set to preferably at least 1 part by weight, and more preferably at least 5 parts by weight, per 100 parts by weight of the base rubber. The upper limit in the amount of inert filler included may be set to preferably not more than 50 parts by weight, more preferably not more than 40 parts by weight, and even more preferably not more than 30 parts by weight. If the amount of inert filler included is too large or too small, a suitable weight and a good hardness and rebound may not be achieved.

In the practice of the invention, from a resource recycling standpoint, one or more type of powder selected from among specific rubber powders (I-a) and (I-b) and a polyurethane resin powder (II) may be included in the rubber ingredients of the core. In this case, such a ground powder or abraded powder may be included in an amount, per 100 parts by weight of the base rubber, which is more than 0, preferably at least 2 wt %, and most preferably at least about 5 wt %. The upper limit is preferably not more than about 40 wt %, more preferably not more than about 35 wt %, even more preferably not more than about 30 wt %, and most preferably not more than about 25 wt %. The rubber powders (I) and the polyurethane resin powder (II) used in the invention may be obtained by Method (i) or Method (ii) below.

Method (i)

Materials obtained by finely grinding, in cases where golf ball covers are formed of a polyurethane resin, the resin from runners discharged as waste during the molding of such golf ball covers as well as flash generated during molding, defectively molded cores, and also the powder obtained when golf balls and golf ball cores are surface ground, can be advantageously used as the specific rubber powders (I-a) and (I-b) and the polyurethane resin powder (II).

Method (ii)

Use can be made of materials obtained by employing a granulator to finely grind defective moldings and golf balls which have been used and discarded, screening the finely ground material, and thereby collecting the specific rubber powders (I-a) and (I-b) and the polyurethane resin powder (II) having particle sizes at or below a given size. When golf balls are granulated, the resulting material may be contaminated with impurities such as paint and ink. However, this material may be directly incorporated into the rubber composition if the amount of such contamination is not large.

The particle sizes of the above rubber powders (I-a) and (I-b) and the polyurethane resin powder (II), expressed as the size of the screen openings, must be set to not more than 3 mm, and may be set to preferably 2 mm or less, more preferably 1.5 mm or less, and even more preferably 1 mm or less. If the particle sizes of the rubber powders (I-a) and (I-b) and the polyurethane resin powder (II) exceed the above-indicated size, the durability of the golf ball may be adversely affected, in addition to which it may not be possible to fully ensure adhesion due to an anchoring effect.

The polyurethane resin powder (II) may be either a thermoplastic polyurethane or a thermoset polyurethane resin, although the use of a thermoplastic polyurethane is more preferred.

The present invention, by including in the core material one or more powder selected from among the two specific rubber powders (I-a) and (I-b) and the polyurethane resin powder (II), confers a suitable surface roughness to the core, thereby making it possible to increase the surface area of contact with the adjoining cover and improve adhesion due to an anchoring effect. In particular, by using a thermoplastic polyurethane in the cover material, the polyurethane resins included in the cover material and the core material melt during molding of the cover material, enabling adhesion between the core and the cover to be increased even further.

Rubber Powder (I-a)

In the invention, the rubber powder (I-a) includes, as an essential ingredient, methacrylic acid or a metal salt thereof. By using (I-a) a rubber powder containing methacrylic acid (MAA) or a metal salt thereof, it is possible to enhance in particular the durability of the golf ball. That is, a material obtained by granulating the above-described core material can be advantageously used as the rubber powder (I-a), in which case the rubber material that is granulated will include methacrylic acid (MAA) or a metal salt thereof as the unsaturated carboxylic acid or a metal salt thereof. The amount of the methacrylic acid or a metal salt thereof which is included in the foregoing rubber powder (I-a) may be set to preferably at least 5 wt %, more preferably at least 10 wt %, and even more preferably at least 15 wt %. The upper limit may be set to preferably not more than 60 wt %, more preferably not more than 50 wt %, even more preferably not more than 40 wt %, and most preferably not more than 30 wt %. If the content is too small, the durability may worsen, and if the content is too large, the rebound may decrease.

Rubber Powder (I-b)

In the invention, the rubber powder (I-b) includes, as an essential ingredient, acrylic acid (AA) or a metal salt of acrylic acid. By using a rubber powder (I-b) containing acrylic acid (AA) or a metal salt of acrylic acid, a good golf ball durability is maintained, in addition to which the initial velocity of the ball is increased, enabling the distance traveled by the ball to be enhanced. That is, a material obtained by granulating the above-described core material can be advantageously used as the rubber powder (I-b), in which case acrylic acid (AA) or a metal salt thereof is included as an unsaturated carboxylic acid or a metal salt thereof in the rubber material that is granulated. Examples of metal salts of acrylic acid include zinc acrylate (ZDA), magnesium acrylate, sodium acrylate, potassium acrylate, aluminum acrylate and calcium acrylate. The content of the acrylic acid or a metal salt thereof which is included in the rubber powder (I-b) may be set to preferably at least 3 wt %, more preferably at least 10 wt %, and even more preferably at least 15 wt %. The upper limit may be set to preferably not more than 60 wt %, more preferably not more than 50 wt %, even more preferably not more than 40 wt %, and most preferably not more than 30 wt %. If the content is too low, the durability may be inferior, and if the content is too high, the rebound may decrease.

Polyurethane Resin Powder (II)

When use is made of the above-described thermoplastic polyurethane powder, it is preferable to use such a powder having a flow starting point of at least 150° C. The flow starting point is more preferably at least 160° C., and even more preferably at least 170° C. The upper limit is preferably not more than 320° C., more preferably not more than 300° C., and even more preferably not more than 280° C. If the flow starting point is too low, the powder will end up melting at the time of core vulcanization, which may result in a loss of core durability and symmetry. On the other hand, if the flow starting point of the powder is too high, it will not be possible to melt the polyurethane at the surface during molding of the cover, as a result of which an additional durability improving effect arising from the use of a thermoplastic polyurethane may not be attainable.

The core may be produced by using a known method to vulcanize and cure a rubber composition containing the various above ingredients. For example, production may be carried out by using a mixing apparatus such as a Banbury mixer or a roll mill to mix the rubber composition, compression molding or injection molding the mixed composition in a core mold, then heating and curing the molded body at a temperature sufficient for the organic peroxide and co-crosslinking agent to act, thereby giving a core having a specific hardness profile. Although the vulcanization conditions are not subject to any particular limitation, the vulcanization temperature is generally from about 100° C. to about 200° C., with the lower limit being preferably at least 150° C., and more preferably at least 155° C., and the upper limit being preferably not more than 180° C., more preferably not more than 175° C., and most preferably not more than 170° C. The vulcanization time is generally in a range of about 10 to about 40 minutes, with the lower limit being preferably at least 12 minutes and the upper limit being preferably not more than 30 minutes, more preferably not more than 25 minutes, and most preferably not more than 20 minutes. The core hardness profile in the invention is achievable through a combination of the vulcanization conditions with preparation of the rubber formulation.

The core diameter, although not subject to any particular limitation, is typically at least 38.0 mm, preferably at least 38.9 mm, and more preferably at least 39.3 mm. The upper limit is preferably not more than 42.1 mm, and more preferably not more than 41.1 mm. At a core diameter outside of this range, the durability of the ball to cracking may worsen dramatically or the initial velocity of the ball may decrease.

It is recommended that the core have a specific gravity of at least 1.05, preferably at least 1.08, and more preferably at least 1.1, but not more than 1.2, preferably not more than 1.15, and more preferably not more than 1.13.

The core deflection (CH) under loading, i.e., the deflection by the core when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), is at least 2.5 mm, preferably at least 2.6 mm, and more preferably at least 2.7 mm. The upper limit in the core deflection is not more than 7.0 mm, preferably not more than 6.0 mm, more preferably not more than 5.5 mm, and most preferably not more than 5.2 mm. If the core deflection (CH) is too small, the feel of the golf ball at impact may be so hard as to make the ball unpleasant to use. On the other hand, if the core deflection is too large, the feel of the golf ball at impact may be so soft as to make the ball unpleasant to use, in addition to which the productivity may decline considerably.

The core rebound (CV) is typically at least 60 m/s, preferably at least 63 m/s, more preferably at least 66 m/s, and most preferably at least 67 m/s. The upper limit is preferably not more than 73 m/s, more preferably not more than 72.5 m/s, and even more preferably not more than 72 m/s. At a core rebound outside of this range, the distance of the ball may dramatically decline or the ball may travel too far, making proper control of the ball impossible. As used herein, “core rebound” is synonymous with core initial velocity.

In the present invention, as shown in the schematic diagram of the core in FIG. 2, letting A be the JIS-C hardness at the surface of the core, B be the JIS-C hardness at a position 2 mm inside the core surface, C be the JIS-C hardness at a position 5 mm inside the core surface, D be the JIS-C hardness at a position 10 mm inside the core surface, E be the JIS-C hardness at a position 15 mm inside the core surface, and F be the JIS-C hardness at the center of the core, it is preferable for the respective values A to F to fall within the specific ranges indicated below. By thus setting the hardness profile at the core interior within specific ranges, both a comfortable feel at impact close to that of a game ball and a good durability to cracking can be obtained.

Letting A be the JIS-C hardness at the surface of the core, the value of A is preferably at least 60, more preferably at least 63, and even more preferably at least 65. The upper limit is preferably not more than 88, more preferably not more than 86, and even more preferably not more than 84.

Letting B be the JIS-C hardness at a position 2 mm inside the core surface, the value of B is preferably at least 54, more preferably at least 57, and even more preferably at least 59. The upper limit is preferably not more than 83, more preferably not more than 81, and even more preferably not more than 79.

Letting C be the JIS-C hardness at a position 5 mm inside the core surface, the value of C is preferably at least 56, more preferably at least 59, and even more preferably at least 61. The upper limit is preferably not more than 85, more preferably not more than 83, and even more preferably not more than 81.

Letting D be the JIS-C hardness at a position 10 mm inside the core surface, the value of D is preferably at least 54, more preferably at least 57, and even more preferably at least 60. The upper limit is preferably not more than 80, more preferably not more than 78, and even more preferably not more than 76.

Letting E be the JIS-C hardness at a position 15 mm inside the core surface, the value of E is preferably at least 51, more preferably at least 54, and even more preferably at least 57. The upper limit is preferably not more than 75, more preferably not more than 73, even more preferably not more than 71, and most preferably not more than 70.

Letting F be the JIS-C hardness at the center of the core, the value of F is preferably at least 48, more preferably at least 51, and even more preferably at least 54. The upper limit is preferably not more than 72, more preferably not more than 70, and even more preferably not more than 68.

Moreover, it is preferable for the hardness profile of the core to satisfy the hardness relationship A>B<C≧D>E>F, for the value A-F to be not more than 19, for the core to be formed in such a way that A has the highest hardness among A to F, and for the value A-C to be from 0 to 8. If the above conditions are not satisfied, the ball may have a diminished feel at impact and a reduced durability to cracking.

The value of A-C is typically from 0 to 8. The lower limit for this value is preferably greater than 0, more preferably at least 1, and even more preferably at least 2. The upper limit is preferably not more than 8, more preferably not more than 6, and even more preferably not more than 4. The value of A-F has a lower limit of preferably at least 2, more preferably at least 4, and even more preferably at least 6. The upper limit in this value is preferably not more than 19, more preferably not more than 18, and even more preferably not more than 17.

In the practice of the invention, the core may be subjected to surface treatment with a solution containing a haloisocyanuric acid and/or a metal salt thereof.

Prior to surface-treating the core with a solution containing a haloisocyanuric acid and/or a metal salt thereof, adhesion between the core surface and the adjoining cover material can be further enhanced by subjecting the surface of the core to grinding treatment (“surface grinding”).

Such grinding treatment removes the skin layer from the surface of the vulcanized core, and thus makes it possible to both enhance the ability of the solution of haloisocyanuric acid and/or a metal salt thereof to penetrate the core surface and also to increase the surface area of contact with the adjoining cover material. Exemplary surface grinding methods include buffing, barrel grinding and centerless grinding.

The haloisocyanuric acid and metal salts thereof are compounds of the following formula (I).

In the formula, X is a hydrogen atom, a halogen atom or an alkali metal atom. At least one occurrence of X is a halogen atom. Preferred halogen atoms include fluorine, chlorine and bromine, with chlorine being especially preferred. Preferred alkali metal atoms include lithium, sodium and potassium.

Illustrative examples of the haloisocyanuric acid and/or a metal salt thereof include chloroisocyanuric acid, sodium chloroisocyanurate, potassium chloroisocyanurate, dichloroisocyanuric acid, sodium dichloroisocyanurate, sodium dichloroisocyanurate dihydrate, potassium dichloroisocyanurate, trichloroisocyanuric acid, tribromoisocyanuric acid, dibromoisocyanuric acid, bromoisocyanuric acid, sodium and other salts of dibromisocyanuric acid, as well as hydrates thereof, and difluoroisocyanuric acid. Of these, chloroisocyanuric acid, sodium chloroisocyanurate, potassium chloroisocyanurate, dichloroisocyanuric acid, sodium dichloroisocyanurate, potassium dichloroisocyanurate and trichloroisocyanuric acid are preferred because they are readily hydrolyzed by water to form acid and chlorine, and thus play the role of initiating addition reactions to the double bonds on the diene rubber molecules. The use of trichloroisocyanuric acid provides an especially outstanding adhesion-improving effect.

The haloisocyanuric acid and/or a metal salt thereof is preferably dissolved in water or an organic solvent and used as a solution.

When water is used as the solvent, the content of the haloisocyanuric acid and/or a metal salt thereof in the treatment solution, although not subject to any particular limitation, may be set to preferably at least 0.5 part by weight, more preferably at least 1 part by weight, and even more preferably at least 3 parts by weight, per 100 parts by weight of water. If the content of haloisocyanuric acid and/or a metal salt thereof is too low, the adhesion improving effect expected after core surface treatment may not be obtained and the durability to impact may be poor. The upper limit is the saturated solution concentration. However, from the standpoint of cost effectiveness, it is preferable to set the upper limit to about 10 parts by weight per 100 parts by weight of water. The core is immersed in the treatment solution for a length of time which, although not subject to any particular limitation, may be set to preferably at least 0.3 second, more preferably at least 3 seconds, and even more preferably at least 10 seconds. The upper limit is preferably not more than 5 minutes, and more preferably not more than 4 minutes. If the immersion time is too short, the anticipated treatment effects may not be obtained, whereas if the immersion time is too long, a loss in productivity may occur.

In cases where use is made of an organic solvent, the solvent may be a known organic solvent, with the use of an organic solvent that is soluble in water being especially preferred. Examples include ethyl acetate, acetone and methyl ethyl ketone. Of these, acetone is especially preferred on account of its ability to penetrate the core surface. The use of a water-soluble solvent is preferred for a number of reasons. For example, such solvents readily take up moisture, the moisture which has been taken up readily undergoing a hydrolysis reaction with the haloisocyanuric acid and/or a metal salt thereof adhering to the core surface. Another reason is that, when water washing is used in a subsequent step, the affinity of water to the core surface increases, along with which a hydrolysis reaction between the water and the haloisocyanuric acid and/or a metal salt thereof more readily arises.

When dissolved in an organic solvent, the content of the haloisocyanuric acid and/or a metal salt thereof in the solution is preferably at least 0.3 wt %, more preferably at least 1 wt %, and even more preferably at least 2.5 wt %. At less than 0.3 wt %, the adhesion improving effect anticipated following core surface treatment may not obtained, which may result in a poor durability to impact. The upper limit in the content may be as high as the saturated solution concentration. However, from the standpoint of cost effectiveness, when prepared as an acetone solution, for example, setting the upper limit in content to about 10 wt % is preferred. The core is immersed in the solution for a length of time which, although not subject to any particular limitation, is preferably at least 0.3 second, more preferably at least 3 seconds, and even more preferably at least 10 seconds. The upper limit is preferably not more than 5 minutes, and more preferably not more than 4 minutes. If the immersion time is too short, the desired effects of treatment may not be obtained, whereas if the immersion time is too long, a loss in productivity may occur.

The method of treating the core surface with a haloisocyanuric acid and/or a metal salt thereof is exemplified by methods which involve coating the core surface with a solution of haloisocyanuric acid and/or a metal salt thereof by brushing or spraying on the solution, and methods in which the core is immersed in a solution of the haloisocyanuric acid and/or a metal salt thereof. From the standpoint of productivity and high penetrability of the core surface by the solution, the use of an immersion method is especially preferred.

After the core has been surface treated with a solution containing haloisocyanuric acid and/or a metal salt thereof, it is preferable to wash the surface of the core with water. Water washing of the core surface may be carried out by a method such as running water, spraying, or soaking in a washing tank. However, because the aim here is not merely to wash, but also to initiate and promote the desired treatment reactions, the washing method should be one that is not too vigorous. Accordingly, preferred use may be made of washing by soaking in a washing tank. In such a case, it is desirable to place the cores to be washed from about one to five times in a washing tank that has been filled with fresh water.

Treating the core surface with a haloisocyanuric acid and/or a metal salt thereof greatly improves adhesion between the core surface and the cover. The reason for this is not well understood, but is thought to be as follows.

First, the haloisocyanuric acid and/or a metal salt thereof, together with the solvent, penetrates to the interior of the diene rubber making up the core and approaches the vicinity of the double bonds on the diene rubber backbone. Water then enters the core surface, whereupon the haloisocyanuric acid and/or a metal salt thereof is hydrolyzed by the water, releasing the halogen. The halogen attacks a double bond on the diene rubber backbone located nearby, as a result of which an addition reaction proceeds. In the course of this addition reaction, the liberated isocyanuric acid is added, together with the halogen, to the diene rubber backbone while retaining its cyclic structure. The added isocyanuric acid has three —NHCO— structures on the molecule.

Because —NHCO— structures are thereby conferred to the core surface that has been treated with the haloisocyanuric acid and/or a metal salt thereof, adhesion with the cover material improves further. It is most likely because of this that the durability of the golf ball to impact improves. Moreover, when a polyurethane elastomer or polyamide elastomer having the same —NHCO— structures on the polymer molecules is used as the cover material, the affinity increases even further, presumably increasing the durability to impact.

Following surface treatment, when the material at the surface portion of the solid core is examined by differential scanning calorimetry (DSC), no exothermic or endothermic peaks are observed from room temperature to 300° C. This means that the functional groups which have been introduced maintain a stable state within this temperature range. In other words, during molding of the cover material, the functional groups which have been introduced do not undergo degradation or the like due to heat, and thus continue to be effective. Also, because melting in the manner of a hot melt resin does not arise, deleterious effects on durability and quality of appearance, such as resin bleed out to the parting line, do not occur. In addition, the very fact that the material in the surface portion of the solid core following the surface treatment described above is stable may be regarded as evidence that the isocyanuric acid having a melting point above 300° C. has been added with its molecular structure still intact.

In cases where, using an organic solvent, the addition of isocyanuric acid and chlorine to the surface of diene rubber has occurred, changes in the bonding states before and after addition appear in an infrared absorption spectrum as increases in the C═O bond (stretching) absorption peak at 1725 to 1705 cm⁻¹, the broad N—H bond (stretching) absorption peak at 3450 to 3300 cm⁻¹, and the C—Cl bond absorption peak at 800 to 600 cm⁻¹. Hence, by measuring the IR absorption spectrum of a surface-treated core and confirming increases in these absorption peaks, it is possible to qualitatively confirm that isocyanuric acid and chlorine addition to diene rubber molecules at the core surface has indeed occurred.

Next, the material making up the cover which directly encases the core is described.

In this invention, a thermoplastic resin such as an ionomer resin or polyurethane may be used as the resin component of the cover. In particular, the use of a resin material composed primarily of polyurethane is preferred. Specifically, use may be made of a thermoplastic polyurethane elastomer or a thermoset polyurethane resin, with the use of a thermoplastic polyurethane elastomer being especially preferred.

The thermoplastic polyurethane elastomer has a structure composed of soft segments formed from a polymeric polyol (polymeric glycol) and hard segments formed from a chain extender and a diisocyanate. Here, the polymeric polyol serving as a starting material may be any which has hitherto been used in the art relating to thermoplastic polyurethane materials, and is not subject to any particular limitation. Exemplary polymeric polyols include polyester polyols and polyether polyols. Polyether polyols are more preferable than polyester polyols because they enable thermoplastic polyurethane materials having a high rebound resilience and excellent low-temperature properties to be synthesized. Illustrative examples of polyether polyols include polytetramethylene glycol and polypropylene glycol. Polytetramethylene glycol is especially preferred from the standpoint of the rebound resilience and the low-temperature properties. The polymeric polyol has an average molecular weight of preferably between 1,000 and 5,000. To synthesize a thermoplastic polyurethane material having a high rebound resilience, an average molecular weight of between 2,000 and 4,000 is especially preferred.

The chain extender employed is preferably one which has hitherto been used in the art relating to thermoplastic polyurethane materials. Illustrative examples include, but are not limited to, 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. The average molecular weight of these chain extenders is preferably between 20 and 15,000.

The diisocyanate employed is preferably one which has hitherto been used in the art relating to thermoplastic polyurethane materials. Illustrative examples include, but are not limited to, aromatic diisocyanates such as 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate and 2,6-toluene diisocyanate, and aliphatic diisocyanates such as hexamethylene diisocyanate. Depending on the type of isocyanate, control of the crosslinking reaction during injection molding may be difficult. In this invention, the use of 4,4′-diphenylmethane diisocyanate, which is an aromatic diisocyanate, is most preferred.

A commercial product may be advantageously used as the thermoplastic polyurethane material composed of the above materials. Illustrative examples include those available under the trade names Pandex T8180, Pandex T8195, Pandex T8290, Pandex T8295 and Pandex T8260 (all available from DIC Bayer Polymer, Ltd.), and those available under the trade names Resamine 2593 and Resamine 2597 (available from Dainichiseika Color & Chemicals Mfg. Co., Ltd.).

The above polyurethane, although not subject to any particular limitation, is preferably a material which is capable of melt-bonding with the above-described polyurethane resin powder (II); a material which, like the polyurethane resin powder (II), is a thermoplastic resin is preferred because melt bonding can be expected to occur. The use of a polyurethane having a high isocyanate content is especially preferred, and makes it possible to improve adhesion with the core material.

The cover has a thickness which is preferably at least 0.3 mm, more preferably at least 0.5 mm, and even more preferably at least 0.7 mm. The upper limit is preferably not more than 2.5 mm, more preferably not more than 2.1 mm, even more preferably not more than 1.9 mm, and most preferably not more than 1.7 mm. If the cover thickness is larger than the above range, the ball rebound may decrease and the flight performance may worsen. On the other hand, if the cover thickness is smaller than the above range, the durability to cracking may decrease. In particular, when the ball is hit thin, or “topped,” the cover may tear.

The cover has a specific gravity which is preferably at least 1.13, more preferably at least 1.14, and even more preferably at least 1.15. The upper limit is preferably not more than 1.30, more preferably not more than 1.20, and even more preferably not more than 1.17.

The cover has a material hardness, expressed as the Shore D hardness, of preferably at least 30, more preferably at least 35, and even more preferably at least 38. The upper limit is preferably not more than 57, more preferably not more than 54, even more preferably not more than 51, and most preferably not more than 50. If the Shore D hardness of the cover is higher than the above range, the appearance performance in long-term use (durability of markings) may decline, in addition to which the flight performance may markedly decrease. On the other hand, if the Shore D hardness of the cover is lower than the above range, the durability to cracking may markedly decrease and, particularly when the ball is topped, the cover may tear. In addition, the spin rate may become very high, possibly shortening the distance traveled by the ball.

In the invention, “Shore D hardness” refers to the hardness measured with a type D durometer in accordance with JIS K 7215 (Durometer D hardness).

The breaking strength of the cover resin material may be set to preferably at least 20 MPa, more preferably at least 25 MPa, even more preferably at least 30 MPa, and most preferably at least 35 MPa. The upper limit may be set to preferably not more than 80 MPa, more preferably not more than 75 MPa, even more preferably not more than 70 MPa, and most preferably not more than 65 MPa. The elongation of the cover resin material may be set to preferably at least 150%, more preferably at least 200%, even more preferably at least 250%, and most preferably at least 300%. The upper limit may be set to preferably not more than 600%, more preferably not more than 550%, even more preferably not more than 520%, and most preferably not more than 490%. The breaking strength and elongation (tensile tests) refer to values measured in accordance with JIS K 7311-1995. By using such a cover resin material having a breaking strength and an elongation in the above-indicated ranges, the durability to cracking, durability to surface loss and durability to abrasion desired of a golf ball intended for long-term use can be improved.

The solid golf ball of the invention typically has numerous dimples formed on the surface thereof, each dimple having a spatial volume below a flat plane circumscribed by an edge of the dimple. In the invention, it is critical for the sum of the individual dimple spatial volumes, expressed as a ratio (VR) with respect to the volume of a hypothetical sphere representing the ball were it to have no dimples on the surface thereof, to be set to from 0.95 to 1.7. The lower limit of VR is preferably 1.0, more preferably 1.1, and most preferably 1.2. The upper limit of VR is preferably 1.6, more preferably 1.5, and most preferably 1.45.

Also, although not subject to any particular limitation, the dimples formed on the solid golf ball of the invention preferably satisfy conditions (1) and (2) below. Although satisfying both of the following conditions (1) and (2) at the same time is preferred, it is acceptable for either one of these conditions alone to be satisfied.

First, referring to FIG. 3, as condition (1), it is preferable for the dimples to have a peripheral edge provided with a roundness represented by a radius of curvature R in a range of from 0.5 to 2.5 mm. The lower limit of the radius of curvature R is more preferably 0.55 mm, and even more preferably 0.6 mm, and the upper limit is more preferably 1.8 mm, and even more preferably 1.5 mm.

Next, as condition (2), it is preferable for the ratio ER of a collective number of dimples RA having a radius of curvature R to diameter D ratio (R/D) of at least 20%, divided by the total number of dimples N on the surface of the ball, to be in a range of from 15 to 95%. Here, the ratio R/D is expressed as a percentage (R/D×100%), a larger value indicating a dimple in which the rounded portion of the dimple accounts for a larger proportion of the dimple size and which has a smoother cross-sectional shape. The ratio ER indicates the number of such smooth dimples as a proportion of the total number of dimples; by setting ER in a range of from 15 to 95%, damage to the paint film at dimple edges can be effectively suppressed. The upper limit in the ratio R/D is preferably not more than 60%, and more preferably not more than 40%. The lower limit in the ratio ER is more preferably at least 20%, and even more preferably at least 25%, and the upper limit is more preferably not more than 90%, even more preferably not more than 85%, and most preferably not more than 70%.

In addition, although not subject to any particular limitation, it is preferable for condition (3) below to be satisfied. As condition (3), it is preferable for the ball to have thereon a plurality of dimple types of differing diameter, and for the ratio DER of a combined number of dimples DE obtained by adding together dimples having an own diameter and having an own radius of curvature larger than or equal to a radius of curvature of dimples of larger diameter than the own diameter plus dimples of a type having a largest diameter, divided by the total number N of dimples on the surface of the ball, to be at least 80%.

Generally, at a fixed dimple depth (see FIG. 3), the radius of curvature R representing the roundness provided to the peripheral edges of the dimples is smaller at smaller dimple diameters. However, above condition (3), by such means as adjusting the depth, sets the radius of curvature R representing the roundness of the peripheral edge to be as large as possible even in dimples having a small diameter, thus forming dimples having a smooth cross-sectional shape, and also increases the proportion of such smooth dimples by setting the above ratio DER to at least 80%, in this way more effectively suppressing damage to the paint film. The ratio DER is more preferably at least 85%, even more preferably at least 90%, and most preferably at least 93%. The upper limit in the ratio DER is 100%.

In addition, the dimples on the golf ball of the invention, although not subject to any particular limitation, preferably satisfy conditions (4) to (6) below. Although it is preferable for all of the following conditions (4) to (6) to be satisfied at the same time, it is acceptable for any one of these conditions alone to be satisfied.

As condition (4), it is preferable for the number of dimple types of differing diameter D on the ball to be 3 or more, and more preferable for dimples of at least five types to be formed. In this case, the diameters D of the dimples, although not subject to any particular limitation, are preferably set in a range of from 1.5 mm to 7 mm, the lower limit being more preferably 1.8 mm and the upper limit being more preferably 6.5 mm. The depths of the dimples, although likewise not subject to any particular limitation, are preferably set in a range of from 0.05 mm to 0.35 mm, the lower limit being more preferably 0.1 mm, and more preferably 0.13 mm, and the upper limit being more preferably 0.32 mm, and even more preferably 0.29 mm.

As condition (5), the total number N of dimples on the surface of the ball is preferably not more than 380, and more preferably not more than 350. The total number N of dimples is even more preferably in a range of from 220 to 340.

As condition (6), it is preferable for the dimples to be formed in such a way that the surface coverage SR of the dimples, which is the sum of the individual dimple surface areas, each defined by a flat plane circumscribed by an edge of the dimple (dash-dot line in FIG. 3), expressed as a percentage of the surface area of a hypothetical sphere representing the ball were it to have no dimples on the surface thereof, is from 60 to 74%. At a surface coverage SR greater than 74%, the intervals between neighboring dimples become too narrow, which may make it difficult to provide the dimple edges with a roundness having the radius of curvature specified in above condition (1). On the other hand, at a surface coverage SR below 60%, the aerodynamic performance decreases, as a result of which the distance traveled by the ball may decrease. The surface coverage SR has a lower limit of more preferably 65%, and even more preferably 68%, and an upper limit of more preferably 73%.

In one-piece golf balls, because rubber often has somewhat of a yellow color, a white enamel paint is generally applied as a first coat, following which a clear paint is applied. In the inventive ball, in order to ensure a good appearance, it is preferable to apply a clear paint to the surface of the ball. The resulting clear coat has a thickness at dimple lands (Y) which is at least 10 μm, preferably at least 12 μm, and most preferably at least 13 μm, but is not more than 30 μm, preferably not more than 25 μm, and most preferably not more than 20 μm; and a thickness at dimple edges (Z) which is at least 8 μm, preferably at least 10 μm, and most preferably at least 11 μm, but is not more than 28 μm, preferably not more than 23 μm, and most preferably not more than 18 μm. Also, the ratio of edge areas (Z) to land areas (Y), expressed as a percentage (Z/Y×100), is at least 60%, preferably at least 70%, and most preferably at least 80%, but is not more than 100%, and preferably not more than 95%. Outside the above range, the durability of markings at dimple edges decreases markedly in long-term use.

The ball diameter is typically at least 42 mm, preferably at least 42.3 mm, and more preferably at least 42.67 mm. The upper limit in the ball diameter is preferably not more than 44 mm, more preferably not more than 43.8 mm, even more preferably not more than 43.5 mm, and most preferably not more than 43 mm.

The ball weight is preferably at least 44.5 g, more preferably at least 44.7 g, even more preferably at least 45.1 g, and most preferably at least 45.2 g. The upper limit is preferably not more than 47.0 g, more preferably not more than 46.5 g, and even more preferably not more than 46.0 g.

The ball has, upon initial measurement, a deflection (BH1) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) of preferably at least 2.5 mm, more preferably at least 2.6 mm, and even more preferably at least 2.65 mm. The upper limit is preferably not more than 7.0 mm, more preferably not more than 6.0 mm, even more preferably not more than 5.5 mm, and most preferably not more than 5.0 mm. Also, letting CH and BH1 be, respectively, the deflection by the core and the deflection by the ball when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), the ratio CH/BH1 therebetween is preferably at least 0.95, more preferably at least 0.96, and even more preferably at least 0.97. The upper limit is preferably not more than 1.1, more preferably not more than 1.09, and even more preferably not more than 1.08. If the ratio CH/BH1 is too large, the deflection of the finished ball relative to the deflection of the core will be very small. In this case, because the cover becomes harder, the feel at impact may decrease and the appearance may decline with long-term use. Conversely, if the ratio CH/BH1 is too small, the cover will be very soft, which may significantly lower the durability to cracking and lead to cracking of the cover, particularly when the ball is topped. In addition, the spin rate may undergo a large increase, which may result in a shorter distance of travel by the ball. Here, “upon initial measurement” means when the ball is measured within about 1 month following production of the ball.

Moreover, in the invention, the ball rebound (BV) has an upper limit of preferably not more than 72 m/s, more preferably not more than 71.7 m/s, even more preferably not more than 71.4 m/s, and most preferably not more than 71.2 m/s. The lower limit is preferably at least 60 m/s, more preferably at least 63 m/s, even more preferably at least 66 m/s, and most preferably at least 67 m/s. As used herein, “ball rebound” is synonymous with ball initial velocity.

In the invention, from the standpoint of ensuring durability over an extended period of time, letting the golf ball of the invention have, upon initial measurement, a deflection BH1 (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and an initial velocity BV1 (m/s), and letting the ball also have, when measured again after 350 days of standing following initial measurement, a deflection BH2 (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and an initial velocity BV2 (m/s), the difference BH2−BH1 is preferably not more than 0.2 mm, more preferably not more than 0.15 mm, and even more preferably not more than 0.1 mm. Also, the value BV2−BV1 is preferably not more than 0.3 m/s, more preferably not more than 0.2 m/s, and even more preferably not more than 0.1 m/s. As a result, the quality of the ball appearance and the flight performance are maintained even in long-term use. Here, “upon initial measurement” means when the ball is measured within about 1 month following production of the ball.

As described above, the solid golf ball of the invention is endowed with the properties required of balls intended for long-term use, including excellent durability to cracking, durability of appearance and low-temperature scuff resistance, and also the ability to maintain a stable feel on impact and a stable flight performance over a long period of time.

EXAMPLES

The following Examples and Comparative Examples are provided to illustrate the invention, and are not intended to limit the scope thereof.

Examples 1 to 8, Comparative Examples 1 to 10

Rubber materials formulated as shown in Table 1 below were furnished for the fabrication of solid golf balls in the examples and comparative examples. These rubber compositions were suitably mixed using a kneader or roll mill, then vulcanized under the temperature and time conditions in Table 1 to produce solid cores in the respective examples. Ingredient amounts in the table below are shown in parts by weight.

TABLE 1 parts by weight (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) Core BR01 40 56 45 55 55 100 100 100 95 formulation IR2200 5 5 20 15 5 BR730 100 SL563 55 39 55 SBR0202 25 30 Perhexa C-40 0.6 0.6 (40% dilution) Actual amount 0.24 0.24 of addition Percumyl D 0.8 0.8 0.8 0.9 0.9 1.2 0.8 0.6 0.6 0.8 Zinc oxide 12.3 12.8 14.8 23 22.5 23.5 23 9.5 9.5 23 Barium sulfate 9.6 9.1 7 Antioxidant 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.2 Methacrylic acid 12 12.5 14.5 25.5 17 29 18 22.5 Zinc 26 methacrylate Zinc acrylate 26 Titanium oxide 4 Vulcanization Temperature (° C.) 170 170 170 170 170 170 170 160 160 170 conditions Time (minutes) 20 20 20 20 20 20 20 13 13 30

Details on the ingredients used in the core formulations in the above table are provided below.

-   BR01: A butadiene rubber synthesized with a nickel catalyst,     available from JSR Corporation; Mooney viscosity ML, 46 -   IR2200: An isoprene rubber available from JSR Corporation; Mooney     viscosity ML, 82 -   BR730: A butadiene rubber synthesized with a neodymium catalyst,     available from JSR Corporation; Mooney viscosity ML, 55 -   SL563: A solution-polymerized styrene-butadiene rubber available     from JSR Corporation; styrene bond content, 20% -   SBR0202: An emulsion-polymerized styrene-butadiene rubber available     from JSR Corporation; styrene bond content, 46% -   Perhexa C-40: An organic peroxide available from NOF Corporation;     1,1-bis(t-butylperoxy)-cyclohexane. Because “Perhexa C-40” is a 40%     dilution, the actual amount of addition is also mentioned in the     tables. -   Percumyl D: An organic peroxide, available from NOF Corporation;     dicumyl peroxide -   Zinc oxide: Available from Sakai Chemical Co., Ltd. -   Antioxidant: “Nocrac NS-6,” available from Ouchi Shinko Chemical     Industry Co., Ltd. -   Methacrylic acid: Available from Kuraray Co., Ltd. -   Zinc methacrylate: Available from Asada Chemical Industry Co., Ltd. -   Zinc acrylate: Available from Nihon Jyoryu Kogyo Co., Ltd. -   Titanium oxide: Available from Ishihara Sangyo Kaisha, Ltd.

In each example, after the rubber composition formulated from the ingredients shown in Table 1 was molded and vulcanized to form a core, the surface of the core was abraded to a desired diameter. Next, surface treatment of the core was carried out by immersing the core for 30 seconds in an acetone solution of trichloroisocyanuric acid (concentration, 3 wt %), then washing the surface of the core with water. The core was then set in a mold for injection molding the cover, and the cover composition shown in Table 2 below was injection molded over the solid core.

TABLE 2 A B C Formulation (pbw) Himilan 1557 50 Himilan 1601 50 Himilan AM7327 50 Surlyn 6320 50 Pandex T8260 Pandex T8195 100 Magnesium stearate 1 1 Titanium dioxide 3.5 2.1 2.1 Polyethylene wax 1.5

Details on the ingredients in the above table are provided below.

-   Himilan: Ionomer resins available under this trade name from     DuPont-Mitsui Polychemicals Co., Ltd. -   Pandex: Thermoplastic polyurethane elastomers available under this     trade name from DIC Bayer Polymer, Ltd. -   Surlyn: An ionomer resin available under this trade name from E.I.     DuPont de Nemours & Co. -   Magnesium stearate: Available from NOF Corporation -   Titanium dioxide: Available under the trade name “Tipaque R550” from     Ishihara Sangyo Kaisha, Ltd. -   Polyethylene wax: Available under the trade name “Sanwax 161P” from     Sanyo Chemical Industries, Ltd.

In order to form a predetermined dimple pattern on the surface of the cover, a plurality of protrusions corresponding to the dimple pattern were formed in the mold cavity, by means of which dimples were impressed onto the surface of the cover at the same time that the cover was injection-molded. Details on the dimples are given below in Table 3. The markings shown in FIG. 5 were printed on the ball surface. In addition, the ball was clear-coated with a paint composed of 100 parts by weight of polyester resin (acid value, 6; hydroxyl value, 168) (solids)/butyl acetate/propylene glycol monomethyl ether acetate (PMA) in a weight ratio of 70/15/15 as the base; 150 parts by weight of a non-yellowing polyisocyanate, specifically an adduct of hexamethylene diisocyanate (available from Takeda Pharmaceutical Co., Ltd. as Takenate D-160N; NCO content, 8.5 wt %; solids content, 50 wt %) as the curing agent; and 150 parts by weight of butyl acetate. In Comparative Example 10, a coating of white enamel paint was applied as a base coat for clear coating.

TABLE 3 Dimple Diameter D Depth R R/D N RA ER DE DER SR VR No. Number (mm) (mm) (mm) ratio (number) (number) (%) (number) (%) (%) (%) Configuration Dimple I 1 24 4.4 0.263 0.65 15 338 102 30 330 98 72 1.31 FIG. 4 2 204 4.2 0.252 0.65 15 3 66 3.6 0.231 0.75 21 4 12 2.7 0.170 0.8 30 5 24 2.5 0.154 0.8 32 6 8 3.4 0.160 0.45 13 Dimple II 1 24 4.4 0.287 0.6 14 338 102 30 330 98 72 1.41 FIG. 4 2 204 4.2 0.274 0.6 14 3 66 3.6 0.249 0.72 20 4 12 2.7 0.180 0.75 28 5 24 2.5 0.154 0.8 32 6 8 3.4 0.160 0.45 13 Dimple III 1 24 4.4 0.216 0.5 11 338 36 11 306 91 72 0.99 FIG. 4 2 204 4.2 0.209 0.5 12 3 66 3.6 0.194 0.6 17 4 12 2.7 0.151 0.6 22 5 24 2.5 0.116 0.5 20 6 8 3.4 0.160 0.5 15

The abbreviations and symbols relating to dimples which appear in Table 4 are explained below.

-   R: Radius of curvature representing roundness provided at the     peripheral edge of a dimple -   R/D ratio: Ratio of radius of curvature R to diameter D -   N: Total number of dimples -   RA: Collective number of dimples having an R/D ratio of at least 20% -   ER: Ratio of RA to total number of dimples N -   DE: Sum of the number of dimples having an own larger than or equal     to a radius of curvature of dimples of larger diameter than the own     diameter, plus the number of dimples of a type having a largest     diameter -   DER: Ratio of DE to the total number of dimples N -   SR: Sum of individual dimple surface areas, each defined by a flat     plane circumscribed by an edge of the dimple, expressed as a     percentage of the surface area of a hypothetical sphere representing     the ball were the ball to have no dimples on the surface thereof. -   VR: Sum of individual dimple spatial volumes, each formed below a     flat plane circumscribed by an edge of the dimple, expressed as a     percentage of the volume of a hypothetical sphere representing the     ball were the ball to have no dimples on the surface thereof

The physical properties of the cores and covers in the respective examples of the invention and the comparative examples, and the physical properties, distance, durability and feel of the solid golf balls obtained in each example were measured or evaluated as described below. The results are presented in Tables 4 and 5.

Deflection of Core and Finished Ball (mm)

The deflection (mm) of the core or finished ball as the test sphere when compressed at a rate of 10 mm/min under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) was measured. The test was performed using a model 4204 test system from Instron Corporation.

Cross-Sectional Hardness of Core

The core was cut with a fine cutter and the JIS-C hardnesses at above positions B to F were measured (at two places in each of N=5 samples) in accordance with JIS K 6301-1975 after holding the core isothermally at 23±1° C.

Surface Hardness of Core

JIS-C hardness measurements were carried out on the core surface (at two places in each of N=5 samples) in accordance with JIS K 6301-1975 after holding the core isothermally at 23±1° C.

Rebound (Initial Velocity) of Core and Finished Ball

The initial velocity was measured using an initial velocity measuring apparatus of the same type as the USGA drum rotation-type initial velocity instrument approved by the R&A. The cores or balls used as the samples were held isothermally at a temperature of 23±1° C. for at least 3 hours, then tested in a room temperature (23±2° C.) chamber. Ten balls were each hit twice, and the time taken for the cores or balls to traverse a distance of 6.28 ft (1.91 m) was measured and used to compute the initial velocity.

Cover Material Hardness

A cover sheet was formed and, after holding the samples isothermally at 23±1° C., the Shore D hardness was measured in accordance with ASTM D-2240.

Breaking Strength and Elongation of Cover Material (Tensile Tests)

The material was formed into a 2 mm thick sheet, and held in a 23±1° C. atmosphere for two weeks. This sample was shaped into dumbbell-shaped test specimens in accordance with JIS K 7311-1995, and the specimens were subjected to measurement in a 23±2° C. atmosphere at a test rate of 5 mm/s, also in accordance with JIS K 7311-1995. The average breaking strength and elongation of each material were calculated from the measured values for five specimens.

Measurement of Coating Thickness

-   -   Lands (Y): The thickness of the clear coat at land areas at         intermediate positions between dimples was measured.     -   Edges (Z): The thickness of the clear coat at dimple edge areas         was measured.

The above measurements were carried out at three places on each of two balls in the respective examples, and the average of these measurements was determined.

Distance

A TourStage X-Drive 701 (loft angle, 9°), manufactured by Bridgestone Sports Co., Ltd., was mounted as the driver (W #1) on a golf swing robot and the ball was struck at a head speed (HS) of 45 m/s. Both the spin rate of the ball immediately after impact and the total distance traveled by the ball were measured.

In addition, after the durability of markings test described below had been carried out, the total distance of the ball was again measured.

Durability to Cracking

The durability of the golf ball to cracking was evaluated using an ADC Ball COR Durability Tester produced by Automated Design Corporation (U.S.). This tester functions so as to fire a golf ball pneumatically and cause it to repeatedly strike two metal plates arranged in parallel. The incident velocity against the metal plates was set at 43 m/s. The number of shots required until cracking of the golf ball arose was measured, and the average value for five golf balls (N=5) was determined.

Low-Temperature Scuffing

The balls were prepared by being held isothermally at a temperature of 0±1° C. for at least 3 hours. Two types of clubs were used: a TourStage X-Wedge 52° and a 2008 TourStage VIQ pitching sand wedge 50° with titanium face, both manufactured by Bridgestone Sports Co., Ltd. Each club was mounted in turn on a golf swing robot and used to strike the ball at a head speed (HS) of 33 m/s, following which the ball was visually examined. The ball appearance was rated according to the following 5-point scale (results shown in the table are average values for the two types of clubs).

-   -   5: Substantially no damage.     -   4: Slight damage was apparent on surface, but was of minimal         concern.     -   3: Surface damage was of concern, but reuse of ball was         possible.     -   2: Surface was damaged with some fraying, although reuse of ball         was marginally possible.     -   1: Surface was frayed, making reuse impossible.

Abrasion Test (Durability of Markings)

Ten golf balls and 3 liters of bunker sand were placed in a magnetic ball mill having an 8 liter capacity and mixing was carried out for 144 hours, following which the balls were visually examined for any loss of markings and to assess the degree of surface scratching and the degree of loss of luster due to abrasion by the sand, as well as the degree of sand adhesion. The ball appearance was rated as “Good,” “Fair” or “NG.”

Feel

Ten teaching professionals hit the test balls with a driver (W #1) and rated the feel of the balls on impact as Good, somewhat hard (Fair), or too hard (NG).

TABLE 4 Example 1 2 3 4 5 6 7 8 Core Type (1) (1) (1) (2) (2) (2) (3) (3) Diameter, mm 39.9 39.9 39.3 39.9 39.3 39.3 39.9 41.1 Specific gravity 1.12 1.12 1.12 1.12 1.12 1.12 1.12 1.12 Deflection under 10-130 kg 5 5 5 5 5 5 4.2 4.2 compression (CH), mm Rebound (CV), m/s 69.2 69.2 69.3 71.6 71.7 71.7 68.8 68.6 JIS-C hardness at core surface (A) 69 69 69 70 70 70 74 74 JIS-C hardness 64 64 64 65 65 65 69 69 2 mm inside core surface (B) JIS-C hardness 66 66 66 67 67 67 71 71 5 mm inside core surface (C) JIS-C hardness 64 64 64 66 66 66 68 68 10 mm inside core surface (D) JIS-C hardness 59 59 59 62 62 62 63 63 15 mm inside core surface (E) JIS-C hardness at core center (F) 54 54 54 58 58 58 57 57 JIS-C hardness difference between core 3 3 3 3 3 3 3 3 surface and 5 mm inside core (A − C) JIS-C hardness difference between 15 15 15 12 12 12 17 17 core surface and center (A − F) Cover Type A A A A A A A A Shore D hardness 45 45 45 45 45 45 45 45 Breaking strength, MPa 40 40 40 40 40 40 40 40 Elongation, % 360 360 360 360 360 360 360 360 Specific gravity 1.15 1.15 1.15 1.15 1.15 1.15 1.15 1.15 Thickness, mm 1.4 1.4 1.7 1.4 1.7 1.7 1.4 0.8 Finished Deflection under 10-130 kg 4.8 4.8 4.7 4.8 4.7 4.7 4.1 4.1 ball compression 30 days after production (BH1), mm Deflection under 10-130 kg 4.7 4.7 4.6 4.7 4.6 4.6 4 4 compression 350 days after BH1 measurement (BH2), mm Difference between BH1 and BH2, mm −0.1 −0.1 −0.1 −0.1 −0.1 −0.1 −0.1 −0.1 Rebound 30 days 68.9 68.9 68.7 71 70.8 70.8 68.5 68.9 after production (BV1), m/s Rebound 350 days 69 69 68.7 71.1 70.8 70.8 68.6 69 after BV1 measurement (BV2), m/s Difference between BV1 and BV2, m/s 0.1 0.1 0 0.1 0 0 0.1 0.1 Diameter, mm 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 Core initial velocity − 0.3 0.3 0.6 0.6 0.9 0.9 0.3 −0.3 ball initial velocity (CV − BV1), ms Core deflection/ball deflection (CH/BH1) 1.04 1.04 1.06 1.04 1.06 1.06 1.02 1.02 Dimples Type I II I II I II I II Clear Land areas (Y), μm 15 17 15 17 15 17 15 17 coat Edge areas (Z), μm 13 15 13 15 13 15 13 15 thickness Coat thickness ratio (Z/Y × 100), % 88 88 88 88 88 88 88 88 Distance HS 45, driver Spin rate, rpm 2920 2920 2940 2920 2940 2940 3080 3060 (30 days after Total distance, m 192 187 191 194 198 193 190 185 production) HS 45, driver Total distance, m 189 184 188 191 195 190 187 182 (after abrasion test) Distance Total distance, m −3 −3 −3 −3 −3 −3 −3 −3 difference Durability Durability to At incident 930 930 1080 930 1080 1080 1010 860 cracking velocity of 43 m/s Low-temperature HS, 38 m/s 3 3 3 4 4 4 3 3 scuffing (at 0° C.) Abrasion test After 144 hours good good good good good good good good (durability of of sand abrasion markings) Feel Driver good good good good good good good good

TABLE 5 Comparative Example 1 2 3 4 5 6 7 8 9 10 Core Type (4) (5) (5) (5) (5) (5) (6) (8) (9) (10) Diameter, mm 39.9 39.9 39.9 39.9 37.3 42.3 39.9 39.9 39.9 42.7 Specific gravity 1.12 1.12 1.12 1.12 1.12 1.12 1.13 1.12 1.12 1.12 Deflection under 10-130 kg 4 5 5 5 5 5 1.8 3.8 3.8 compression (CH), mm Rebound (CV), m/s 67.7 69.7 69.7 69.7 69.7 69.7 73.7 77 77.4 JIS-C hardness at core surface (A) 71 64 64 64 64 64 90 70 70 80 JIS-C hardness 66 59 59 59 59 59 88 65 65 75 2 mm inside core surface (B) JIS-C hardness 68 61 61 61 61 61 87 68 68 77 5 mm inside core surface (C) JIS-C hardness 66 61 61 61 61 61 81 66 66 71 10 mm inside core surface (D) JIS-C hardness 63 59 59 59 59 59 74 62 62 67 15 mm inside core surface (E) JIS-C hardness at core center (F) 59 55 55 55 55 55 70 58 58 63 JIS-C hardness difference between core 3 3 3 3 3 3 3 2 2 3 surface and 5 mm inside core (A − C) JIS-C hardness difference between 12 9 9 9 9 9 20 12 12 17 core surface and center (A − F) Cover Type A A B C A A A A A Shore D hardness 45 45 60 45 45 45 45 45 45 Breaking strength, MPa 40 40 17 12 40 40 40 40 40 Elongation, % 360 360 100 120 360 360 360 360 360 Specific gravity 1.15 1.15 0.99 0.99 1.15 1.15 1.15 1.15 1.15 Thickness, mm 1.4 1.4 1.4 1.4 2.7 0.2 1.4 1.4 1.4 Finished Deflection under 10-130 kg 3.8 4.75 4.3 4.85 4.35 5 1.9 3.8 3.8 3.1 ball compression 30 days after production (BH1), mm Deflection under 10-130 kg 3.7 4.65 4.2 4.75 4.25 5 1.9 3.5 3.5 3.1 compression 350 days after BH1 measurement (BH2), mm Difference between BH1 and BH2, mm −0.1 −0.1 −0.1 −0.1 −0.1 0 0 −0.3 −0.3 0 Rebound 30 days 67.3 69.1 68.9 68.5 68.1 69.4 73 76 76.4 74.6 after production (BV1), m/s Rebound 350 days 67.4 69.1 69.1 68.6 68.1 69.4 73 75.1 75.5 74.7 after BV1 measurement (BV2), m/s Difference between BV1 and BV2, m/s 0.1 0 0.2 0.1 0 0 0 −0.9 −0.9 0.1 Diameter, mm 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 Core initial velocity − 0.4 0.6 0.8 1.2 1.6 0.3 0.7 1.0 1.0 ball initial velocity (CV − BV1), ms Core deflection/ball deflection (CH/BH1) 1.05 1.05 1.16 1.03 1.15 1.00 0.95 1.00 1.00 Dimples Type I I I I III I I I I I Clear Land areas (Y), μm 15 15 15 15 17 15 15 15 15 15 coat Edge areas (Z), μm 13 13 13 13 8 13 13 13 13 13 thickness Coat thickness ratio (Z/Y × 100), % 88 88 88 88 47 88 88 88 88 88 Distance HS 45, driver Spin rate, rpm 3180 2960 2900 3050 2610 3210 3480 3100 3070 3650 (30 days after Total distance, m 187 193 192 190 194 193 212 225 227 219 production) HS 45, driver Total distance, m 184 190 185 184 187 190 209 222 224 213 (after abrasion test) Distance Total distance, m −3 −3 −7 −6 −7 −3 −3 −3 −3 −6 difference Durability Durability to At incident 1035 930 940 610 1530 630 1425 555 520 620 cracking velocity of 43 m/s Low-temperature HS, 38 m/s 1 1 3 1 1 1 3 4 4 5 scuffing (at 0° C.) Abrasion test After 144 hours good good NG NG NG good good good good NG (durability of of sand abrasion markings) Feel Driver good good NG good good good NG good good good

From the results in Tables 4 and 5, the comparative examples were confirmed, as shown below, to be inferior to the working examples of the invention.

In the golf ball of Comparative Example 1, styrene-butadiene rubber having a high styrene bond content (trade name: SBR0202) was used, as a result of which the scuff resistance at low temperature was poor.

In the golf ball of Comparative Example 2, styrene-butadiene rubber having a high styrene bond content (trade name: SBR0202) was used, as a result of which the scuff resistance at low temperature was poor.

In the golf ball of Comparative Example 3, the cover had a small breaking strength and a small elongation, as a result of which the durability to abrasion was poor and the decrease in flight performance was large. In addition, the cover was hard, as a result of which the feel at impact with a driver was poor.

In the golf ball of Comparative Example 4, the cover had a small breaking strength and a small elongation, as a result of which the durability to abrasion was poor and the decrease in flight performance was large.

In the golf ball of Comparative Example 5, styrene-butadiene rubber having a high styrene bond content (trade name: SBR0202) was used, as a result of which the scuff resistance at low temperature was poor. In addition, the dimple edges had a small radius of curvature R, as a result of which the abrasion durability was poor and the decrease in flight performance was large.

In the golf ball of Comparative Example 6, styrene-butadiene rubber having a high styrene bond content (trade name: SBR0202) was used, as a result of which the scuff resistance at low temperature was poor. In addition, the cover was too thin, as a result of which the durability to cracking decreases.

In the golf ball of Comparative Example 7, the core deflection was very small, as a result of which the feel at impact with a driver was very poor.

In the golf ball of Comparative Example 8, the core included zinc methacrylate. As a result, the changes over time in deflection and rebound were large, in addition to which the durability to cracking was poor.

In the golf ball of Comparative Example 9, the core included zinc acrylate. As a result, the changes over time in deflection and rebound were large, in addition to which the durability to cracking was poor.

The golf ball of Comparative Example 10 had a one-piece construction in which the surface rubber material had a small breaking strength and a small elongation. As a result, the durability to cracking and the durability to abrasion were both poor, and the ball exhibited a large decline in flight performance (the rubber material had a breaking strength of 15 MPa and an elongation of 88%). 

1. A solid golf ball comprising a core and a cover, the core being formed of a rubber composition comprising a base rubber, a co-crosslinking agent, a crosslinking initiator and a metal oxide, wherein the base rubber is a mixture of polybutadiene and a styrene-butadiene rubber, the styrene-butadiene rubber having a styrene bond content of not more than 35 wt %, and the co-crosslinking agent is methacrylic acid; the core has a deflection CH when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) of from 2.5 to 7.0 mm; and the ball has formed on a surface thereof a plurality of dimples, each having a spatial volume below a flat plane circumscribed by an edge of the dimple, the sum of the individual dimple spatial volumes, expressed as a percentage (VR) of the volume of a hypothetical sphere were the ball to have no dimples on the surface thereof, being from 0.95 to 1.7.
 2. The solid golf ball of claim 1, wherein the metal oxide is zinc oxide.
 3. The solid golf ball of claim 1, wherein the polybutadiene accounts for up to 80 wt % of the base rubber in the rubber composition, the styrene-butadiene rubber accounts for between 20 and 80 wt % of the base rubber, and the isoprene rubber accounts for between 0 and 60 wt % of the base rubber; and wherein the rubber composition includes from 6 to 40 parts by weight of methacrylic acid, from 6 to 30 parts by weight of the metal oxide, from 0.3 to 5.0 parts by weight of the crosslinking initiator, and from 0.1 to 1.0 part by weight of the antioxidant per 100 parts by weight of the base rubber.
 4. The solid golf ball of claim 1, wherein the core has a specific gravity of from 1.05 to 1.2.
 5. The solid golf ball of claim 1, wherein the cover is formed of a resin material which is composed primarily of a polyurethane.
 6. The solid golf ball of claim 5, wherein the resin material of the cover is composed primarily of a thermoplastic polyurethane.
 7. The solid golf ball of claim 1, wherein the cover has a material hardness, expressed in terms of Shore D hardness, of from 30 to
 57. 8. The solid golf ball of claim 1, wherein the cover is formed of a resin material having a breaking strength of from 20 to 80 MPa.
 9. The solid golf ball of claim 1, wherein the cover is formed of a resin material having an elongation of from 150 to 600%.
 10. The solid golf ball of claim 1, wherein the cover has a thickness of from 0.3 to 2.5 mm.
 11. The solid golf ball of claim 1, wherein the ball has an initial velocity (BV) of not more than 72 m/s.
 12. The solid golf ball of claim 1, wherein the core has a deflection CH (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), the ball has, upon initial measurement, a deflection BH1 (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and an initial velocity BV1 (m/s), and also has, when measured again after 350 days of standing following initial measurement, a deflection BH2 (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and an initial velocity BV2 (m/s), such that: BH1 is from 2.5 to 7.0 mm, the ratio CH/BH1 is from 0.95 to 1.1, the difference BH2−BH1 is not more than 0.2 mm, and the difference BV2−BV1 is not more than 0.3 m/s.
 13. The solid golf ball of claim 1, wherein the dimples formed on the surface of the ball satisfy conditions (1) to (6) below: (1) the dimples have a peripheral edge provided with a roundness represented by a radius of curvature R of from 0.5 to 2.5 mm; (2) the ratio ER of a collective number of dimples RA having a radius of curvature R to diameter D ratio (R/D) of at least 20%, divided by a total number of dimples N on the surface of the ball, is from 15 to 95%; (3) the ball has thereon a plurality of dimple types of differing diameter, and the ratio DER of a combined number of dimples DE obtained by adding together dimples having an own diameter and an own radius of curvature larger than or equal to a radius of curvature of dimples of larger diameter than said own diameter plus dimples of a type having a largest diameter, divided by the total number of dimples N on the surface of the ball, is at least 80%; (4) the number of dimple types of differing diameter is 3 or more; (5) the total number of dimples N is not more than 380; and (6) the surface coverage SR of the dimples, which is the sum of individual dimple surface areas, each defined by a flat plane circumscribed by an edge of the dimple, expressed as a percentage of the surface area of a hypothetical sphere were the ball to have no dimples on the surface thereof, is from 60 to 74%. 